Hearing problems are a frequent health issue as we age. Recently, a University of Toronto research group, led by Bernhard Ross of the Department of Medical Biophysics in the Temerty Faculty of Medicine, has discovered a mechanism that explains why the elderly frequently have difficulties distinguishing speech from other noise.

The researchers discovered that difficulty in understanding speech in noise (SIN) relates to the changes in central auditory processing caused by decreasing levels of gamma-aminobutyric acid (GABA) in the brain. It has been theorized that GABA is partly responsible for controlling levels of fear or anxiety. Levels of GABA in the brain naturally decrease as we age. The results were published in the journal NeuroImage earlier this year.

The mechanism of how GABA levels affect SIN understanding

GABA is one of the major inhibitory chemicals that exists in all mammals’ central nervous systems. As an inhibitory chemical, its function is to hinder the transmission of signals between neurons. When GABA levels are too high, the abundance of nervous signals can cause seizures or mood disorders. When GABA levels are too low, the sluggish neuron-to-neuron communication can cause sleepiness.

In their study, the researchers began to use non-invasive magnetic resonance spectroscopy to measure the GABA level in the brain. 

“Changes in the central auditory system that contribute to SIN loss are not as well-understood, and therefore much more difficult to correct,” the researchers wrote in a written interview with The Varsity. “We have identified an aging-related change in the central auditory system – a reduction in the GABA level – which was correlated with SIN loss.”

“This finding paves the way for more research into how GABA affects SIN understanding, and suggests we should look into how we might prevent an age-related decline in GABA levels or restore them when they become low,” they added.

However, the researchers also pointed out two subtleties in their result. First, they mentioned that GABA is only one of several factors that contribute to SIN loss. “It is only part of the complex interaction between changes at all levels of the auditory system, from peripheral hearing loss to cognitive deficits, that together cause reduced SIN understanding.”

They also noted the distinction between biological and chronological age. Whereas chronological age is simply how old one is in real time, biological age is a more complicated measure of how the body is performing relative to healthy models. It ranges across everything from immune and inflammatory measures to blood sugar levels — and is changeable with diet and exercise, unlike chronological age.

It is biological age that may actually affect hearing loss. “Individuals with the same chronological age can have very different biological ages,” the researchers wrote. “The process that causes reduced GABA levels is likely a part of normal, healthy aging, and reflects biological age more than chronological age.

The broader context

This paper is a continuation of research that was published last year that also connected declining levels of GABA to SIN loss. Whereas the first paper was more general, this second followup looked specifically at which side of the brain has greater influence on SIN loss. Researchers found stronger correlations between the brain’s right auditory cortex — a region known to be associated with cognitive functioning — and GABA levels than the left cortex.

It comes as part of a growing research program studying the impacts of GABA on hearing loss and cognitive decline, including Alzheimer’s disease (AD).

“The topics of aging, dementia (including AD), and SIN are more closely interrelated than previously thought,” the researchers wrote. They had previously found evidence for a relationship between rhythms in one’s brain activity and “the degree to which noise affects listening and speech understanding.”

“The same type of brain rhythms has been found useful as a biomarker for developing AD. In mice it has been found that stimulating gamma oscillations could delay the progression of AD.”

The hope is that studying GABA will unveil the complicated mechanics of aging and cognitive decline.